The powering requirements of new computer and communications systems continue to require increased flexibility and performance. Increased system flexibility has resulted in increased interest in distributed power architectures. A fundamental requirement for distributed systems is module parallelability and current sharing.
Many different current share techniques have been suggested over the years to parallel multiple power supplies. In most cases, these schemes measure the current provided by each module, amplify it, compare it to the current form of other modules, and adjust the regulation voltage of its own module so as to minimize the difference in the output currents between modules. Many different variations of this technique have been suggested and implemented in industry. See Zhou, X., Peng, X., and Lee, F. C., "A high power density, high efficiency and fast transient voltage regulator module with a novel current sensing and current sharing technique", IEEE APEC 99 Proceedings, pp. 289-294; Petruzziello, F., Ziogas, P. D., and Joss, G., "A novel approach to paralleling of power converters units with true redundancy", IEEE PESC 90 Proceedings, pp. 808-813; Small, K. T., "Single wire current share paralleling of power supplies", U.S. Pat. No. 4,717,8333, 1988; Jordan, M., "Load share IC simplifies power supply design", High Frequency Power Conversion Conf. Proc., pp. 65-76, 1991; Jordan, M., "UC3907 load share IC simplifies parallel power supply design", Application Handbook Unitrode, pp. 3-203-3-212 (U-129), 1997; Balogh, L., "The UC3902 load share controller and its performance in distributed power systems", Application Handbook Unitrode, pp. 3-626-3-633 (U-163), 1997; Jamerson, C., Mullet, C., "Paralleling supplies via various droop methods", High-Frequency Power Conversion Conf. Proc., pp. 68-76, 1994.
There are two widely used solutions of implementing current sharing modules in industry today. The first solution is the droop method. In this method, the system output voltage is allowed to droop as the load current increases, resulting in improved current sharing. The droop method works well if the initial set point of the different modules is relatively similar. However, there are systems which do not permit the voltage to droop.
The second solution, active current sharing, is generally used in these systems. In active current sharing, the individual current of each parallel module is measured, amplified, and compared to each other. Based on the result of this comparison, the module set point is adjusted to drive the difference between the output currents of each module to zero. Many different implementations of active current sharing are being used in industry today. Generally, active current sharing requires the use of several operational amplifiers.
Although the droop method and active current sharing are widely used in industry, each has disadvantages that involve either high cost, increased size, complexity, or decreased system performance. The droop method offers a simple and cost effective solution at the expense of system performance. Active current sharing, on the other hand, produces high performance but must be implemented with complex, costly, and power dissipating circuitry. Thus, what is needed is a cost effective and less complex current share circuit which does not decrease system performance nor significantly increase the physical size of the system.